I know this might will be totally hated on, but I think Togekiss should rise to B-. It's commonly thought of a jack-of-all-trades master-of-none Pokemon, but so is Garchomp, and that's A. Togekiss and Garchomp both share a) an incapability of mastering anything and b) extreme versatility. I'll stop comparing Kiss to Chomp here, because I'm arguing for B-, not A. Basically, while Togekiss has two huge issues (the SR weakness and the Electric-weakness) and one smaller one (competition with fable), it's able to secure some interesting niches. My favorite set of Toge's is the NastyPass set. The set works because of Togekiss's massive bulk, and while its typing isn't the best, it still provides excellent synergy with Electric-types. Thundurus-I already has nasty plot and is immune to ground, so toge is a bit useless for it. However, the other three big offensive electrics (Mega Ampharos, Mega Manectric, and Raikou), all greatly appreciate the sweeping ability and the ground immunity that only Togekiss can provide. Masterclass made an excellent team using Togekiss + Ampharos, and I definitely implore you to check it out. Togekiss's second best set, in my opinion, is its Life Orb set, which is really cool. It again offers defensive synergy, but instead of using Nasty Plot to make its teammates threatening, it uses it with a Life Orb to destroy things. This sets only huge problem is the choice between Fire Blast, Aura Sphere, and Roost (you can only pick two). Otherwise, even with only 80 base speed, LO toge is a really cool set. Then there's the bread and butter specially defensive set, which has a few perks. Fire and foremost, it beats non-Sludge Wave Landorus, which basically should be all Landorus at this point. Other than Landorus, it's also able to serve as an emergency check to Landorus-T, Garchomp, Keldeo, and Mega Gardevoir, while being able to provide hazard removal support, paralysis support for slower teams, offensive presence, and Heal Bell. It can't run all at once, but it's still an interesting defogger capable of beating grounds + less reliably keldeo and gardevoir. It also flinches everything, which allows it to, especially with twave, limit what can beat it.
Electric types are the bane of Togekiss' existence, but it's still able to cope, providing sufficient team support. I think Togekiss' access to Nasty Plot, which is either able to make any special attacker, but specifically Electric types, or itself, extremely dangerous, plus its defensive utility (ground check, garde check, keld check, defog, heal bell, twave) warrants a move up to B-. It's commonly hated on, like Empoleon, but it's not all that different from other B- mons. Ampharos takes the mega slot, needs either togekiss or rain if it's offensive, has to deal with its non mega form (ie it can't switch in on chary if it hasn't evolved yet), and has an absence of reliable recovery. Nevertheless, its unique defensive typing, good coverage, and access to Agility let it carve itself two niches in OU. Togekiss has reliable recovery, the ability to remove rocks, and doesn't cost a mega slot. However it lacks Ampharos' excellent defensive typing and sr neutrality, so I'd say defensively they're about on par. Offensively, Ampharos really needs support to shine (nasty pass or rain), while togekiss doesn't. However, Togekiss is slower, so it's somewhat harder to use v. offense. It also doesn't cause any strain on teambuilding, other than needing a defogger. Again, I'd say their about on par. Then, Toge also has its best set imo, a slow baton passing set capable of making so many pokemon, specifically ampharos, absolutely terrifying. Admittedly it has its flaws, but its not different from Ampharos, Cress, or Chesnaught. All have certain exploitable weaknesses, but have either versatility or extreme proficiency at what they do to make up for it. Togekiss is ridiculously similar to these three; it's versatile as hell, has exploitable weaknesses, and is very good at at least one of its roles, NastyPassing. It's really easy to hate on, since it was so hyped earlier, but I think now it's under hyped.